1. Field of the Invention
The present invention relates to control strategies for engines, and more particularly, to control strategies for rapidly heating an emission control device.
2. Background of the Invention
Emission control devices, such as catalytic converters, can reduce emissions generated by engine combustion. However, the effectiveness of such emission control devices varies with operating temperature. For example, the efficiency of an emission control device may be much greater at higher temperatures than it is at lower temperature. Typically, a xe2x80x9clight-offxe2x80x9d temperature is used to signify a certain temperature above which a prescribed efficiency is achieved.
In vehicle emission control systems, measures are taken to rapidly heat emission control devices after starting of a vehicle, so that tail pipe emissions can be minimized. In particular, as described in U.S. Pat. No. 5,479,745, an ignition timing signal is retarded during cold engine operation to rapidly warm a catalyst converter coupled to the engine exhaust. By retarding ignition timing, additional heat is provided to the exhaust, thereby providing more heat to increase catalyst temperature.
The inventor herein has recognized a problem with approaches such as the one described above. In particular, as emission requirements are continually lowered, attempts are made to add more and more heat to the catalyst to obtain faster and faster catalyst light-off. In particular, the inventor herein has found that as ignition timing is retarded further (to obtain more heat), engine combustion becomes inconsistent. This inconsistency in combustion (due to the very late ignition timing) leads to vibration during idle conditions, which can degrade customer satisfaction. Further, such vibration can also lead to degraded idle speed control.
While the inventor herein has found that increasing idle speed can reduce the above problems, such action has still other problems. For example, operating at a higher engine speed requires additional fuel, which can degrade fuel economy. Also, higher idle speed can also degrade customer satisfaction since additional noise may be generated.
Another measure taken to rapidly heat an emission control device is to operate some cylinders combusting a mixture lean of stoichiometry and others a mixture rich of stoichiometry. When the lean and rich combustion gases mix in the exhaust, the un-burnt reactants from the rich gases react with excess oxygen in the lean gases, thereby generating heat. Such a system is described in U.S. Pat. No. 6,189,316.
However, the inventor herein has recognized problems with this approach also. In particular, the amount of heat generated is typically proportional to the amount of excess oxygen and reactants in the exhaust. To increase heat generated, the lean bank can be run even leaner, and the rich bank even richer. However, the degree of leanness in the lean bank is limited by combustion inconsistency, just as the amount of ignition retard is limited. Thus, attempting to run too lean leads to similar problems as described above, such as engine vibration.
The above disadvantages are overcome by a method for controlling an engine having at least first and second groups of cylinders with the engine coupled to an emission control device. The method comprises providing a request to increase temperature of the emission control device and in response to the request, operating the first group of cylinders to induct air with substantially no injected fuel and operating the second group of cylinders to combust a rich air-fuel mixture. For example, the request for an increase temperature can be for an increase in exhaust temperature or an increase in catalyst temperature.
Since excess oxygen is supplied by cylinders not performing combustion the amount of oxygen in the exhaust gas is not limited by a lean combustion limit. In other words, all of the air passing through the non-combusting cylinders is available for reaction in the exhaust.
An advantage of such an approach is that it is possible to provide heat for rapidly heating an emission control device without the vibration problem caused by excessive ignition timing retard. In other words, operating an engine with some cylinders producing output and others producing substantially no output creates less vibration than operating all cylinders producing output at a retarded ignition timing.
Also, since fewer cylinders are producing all of the engine output, the cylinders combusting are operating at a higher load than if all of the cylinders were combusting. As such, the cylinders combusting a rich mixture are operating at a higher load, which reduces the pumping work of those cylinders. This yields additional fuel economy savings. Further still, since the remaining operating cylinders are at a higher load, they can tolerate additional ignition timing retard. As such, the present invention not only allows for heat generation by mixing reactants with excess oxygen in the exhaust, but also for providing heat from the combusted cylinders with retarded ignition timing.
The combusting of a rich air-fuel ratio can be done in multiple ways. For example, it is possible to directly inject fuel with inducted air, or to have port fuel injection with inducted air. Another way to achieve combustion of a rich air-fuel ratio is to induct a mixture of air and fuel vapor.